片层石墨尺寸对片层石墨/Al复合材料的强度和热导率的影响

Effect of Graphite Flake Size on the Strength and Thermal Conductivity of Graphite Flakes/Al Composites

采用粉末冶金法制备了名义尺寸为150、300、500 mm的片层石墨(graphite flakes,Gf)增强铝基(50%Gf/Al,体积分数)复合材料,得到密度均接近理论密度的致密复合材料坯锭。片层石墨与铝合金基体结合紧密,界面处无裂纹、孔洞等缺陷。片层石墨的(001)Gf基面与复合材料坯锭的圆周方向(坯锭的xy平面)基本平行,但受粉末冶金工艺的影响,较小片层石墨的(001)Gf基面与坯锭的xy平面略有偏差。随着片层石墨的尺寸增大,偏差逐渐减少。复合材料的强度随着片层石墨尺寸增加逐渐降低。150 mm片层石墨复合材料的弯曲强度为82 MPa,当片层石墨尺寸增至500 mm时,强度降低至39 MPa。片层石墨强度较低,裂纹容易沿片层石墨的层间扩展,随着片层石墨尺寸增大,这一现象更加明显,容易在断口中观察到片层石墨剥离的现象。复合材料xy平面的热导率随片层石墨尺寸增大而增加,最高可达604 W/(m·K),与尺寸较小的片层石墨相比提高63%。300、500 mm片层石墨复合材料的界面换热系数略低于理论值,但150 mm片层石墨复合材料的界面换热系数明显小于理论值。除了片层石墨的尺寸,其形状、分布和内部缺陷等对复合材料的热导率也有一定的影响。

Graphite flakes reinforced AI matrix composites （GJAI） with low density, good machining property and high thermal conductivity are considered an excellent heat sink materials used in electronic industry. When the composites are manufactured by liquid method such as liquid infiltration, it is easy to achieve a high thermal conductivity composite. However, the Al4C3 phase would be formed in the composite, which will decrease the corrosion properties of the composites. The powder metallurgy technique could avoid the formation of the Al4C3 phase. In this work, three seized graphite flakes （150, 300, 500 μm） were used to investigate the effect of the graphite flake size on the strength and thermal conductivity of Gf/Al alloy composites. The 50%Gf/Al alloy （volume fraction） composites were fabricated by the powder metallurgy technique. The density of all the three Gf/Al alloy composites were similar to the theoretical density. The graphite flakes had a well bonding with AI alloy matrix without cracks and pores. The （001）Gi basal plane of the graphite flakes were almost parallel to the circular plane （xy plane） of the composites ingot. However, for the small graphite flakes, their （001）Gi basal plane was not well parallel to the xy plane of the composite ingot due to the powder metallurgy process. For the large graphite flakes, they exhibited a good orientation in the xy plane of the composite ingot. The strength of the Gf/Al alloy composites decreased with the increase of the graphite flake size. For the 150 μm graphite flake, the bending strength of the Gf/Al alloy composite was 82 MPa. However, for the 500 μm graphite flake, the bending strength of the composite decreased to 39 MPa. Due to the low strength between the layers of the graphite flake, the cracks were prone to expand in the graphite flake. As the size of the graphite flake increased, this phenomenon became more obviously. It is easy to observe that the graphite flakes peeled off on the fracture surfaces. When the size of the graphite flake increased from 150 μm to 500 μm, the thermal conductivity increased by 63%. The highest thermal conductivity was 604 W/（m·K）. The interfacial thermal conductance （he） of the composites were calculated by the MaxwelI-Garnett type effective medium approximation model. The ho of 300 and 500μm graphite flake Gf/Al alloy composites were slightly lower than the theoretical value （calculated by the acoustic mismatch model）. However, the hc of the 150 μm graphite flake Gf/Al alloy composite was lower than that of the theoretical value. Besides the size of the graphite flakes, the shape, distribution and defect of the graphite flakes also influenced the thermal conductivity of the composites.